A proton decay mode p*nuK^+ has been searched for with a 50,000ton water Cherenkov detector "Super-Kamiokande". A search was carried out specifically by looking for a delayd coincidence signal of a gamma ray and a muon which are produced from an Oxygen decay^<16>O* ^<15>N^<**>+K^++nu, ^<15>N^<**>* ^<15>N+gamma, K^+*mu^++nu.We first made a light emitter which simulates Cherenkov light from the gamma ray and muon taking into account of their energies and angles. The light emitter was then lowered into the Super-Kamiokande detector and the detection efficiency of the gamma ray and muon was studied. It was found that the efficiency was excellent up to 5nsec of delay time, which is sufficient for the present search.We then analyzed the 414 day-data obtained by Super-Kamiokande. The results are the following : We have not found evidence for the proton decay and therefore set the lower limit of proton lifetime divided by the branching ratio (tau/B) ;From the muon energy-spectrum------2.8*10^<32> years,From the Kaon decay pi^++pi^0-----1.3*10^<32> years,From the present analysis gamma+mu---1.5*10^<32> years,These results are the strongest limit on the proton decay p*nuK in the world. The present method will eventually provide the most sensitive limit, since the search is free from background.